Julia Felsenthal is a senior culture writer at Vogue.com. Follow her on Twitter.

No, unless it's someone else's toothbrush (or someone else's cold). Much like the admonition from shampoo companies to "lather, rinse, repeat," the idea that a toothbrush must be replaced seems designed to sell more product. Once you've been infected with a particular strain of a virus, you develop antibodies that make the likelihood of re-infection very low. Even if the virus were still hanging out on your toothbrush after you recovered—colds and flus can survive there in an infective state for anywhere from a few hours to three days—those antibodies should keep you from contracting the same illness twice. Your toothbrush is no more dangerous while you're still sick, since the viral load on the bristles is negligible compared with what's already in your system.

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It is possible to re-infect yourself with bacteria, however. If you were afflicted with strep throat, for example, a colony of streptococcal bacteria might end up on your toothbrush and remain there long enough to give you a second case after you'd taken a course of penicillin. But that threat might be mitigated by toothpaste, which sometimes contains antibacterial compounds. And, despite the claims of toothbrush manufacturers, this wouldn't apply to a typical case of the (viral) sniffles.

It is possible to catch a cold, a bacterial infection, or even a blood-borne disease such as Hepatitis B or C from someone else's toothbrush. * (It's an especially bad idea to use a sick person's toothbrush while the bristles are still wet.) Even if you don't put it in your mouth, the infected implement might contaminate another toothbrush nearby: When two are stored in the same cup, their bristles sometimes come into contact. A dirty toothbrush might also pass bacteria or virus particles to the rim of a toothpaste tube, and then on to another toothbrush from there. Another questionable practice: storing your toothbrush so close to the toilet that spray from the flush can reach its bristles, especially in a shared bathroom. As this episode of Mythbusters points out, the presence of some fecal coliforms on your toothbrush won't necessarily make you sick, but the spray from toilet water has been known to spread noroviruses, which are responsible for outbreaks of gastrointestinal illness on cruise ships and in other places.

Despite all this, the American Dental Association isn't overly concerned over the microorganisms living on your toothbrush bristles. Generally speaking, our immune systems are up to the task of fighting off any illnesses that might result from them. The ADA does suggest rinsing off your toothbrush after brushing, storing it in a position that allows it to air dry, and keeping it away from other toothbrushes. The association also recommends replacing your toothbrush every three to four months, once the bristles are frayed and worn, but not in the aftermath of every cold.

Explainer thanks Kavita Ahluwalia of Columbia University, Alice Boghosian of the American Dental Association, Keith Crandall of Brigham Young University, and Charles Gerba of the University of Arizona.Like Slateand the Explainer on Facebook. Follow us on Twitter.

Correction,Feb. 7, 2011: This article incorrectly stated that Hepatitis A is a blood-borne disease. (Return to the corrected sentence.)